Light waveguide and its manufacturing method

ABSTRACT

When forming a laminated optical waveguide, an extra adhesive layer lies and a wave-guiding mode is present. Moreover, it is difficult to remove the adhesive layer.  
     A first substrate  11  on which an optical-waveguide groove is formed and a second substrate  13  are used, the second substrate  13  is bonded to the plane of the first substrate  11  on which the optical-waveguide groove is formed by a material  12  having a refractive index higher than those of the first substrate  11  and second substrate  13 , the optical-waveguide groove is filled with the material  12 , and the refractive index of the first substrate  11  is different from the refractive index of the second substrate  13.

TECHNICAL FIELD

[0001] The present invention relates to an optical waveguide mainly usedfor optical communication and a method of forming the same.

BACKGROUND ART

[0002] In recent years, an optical communication system using opticalcommunication having a wide frequency band and added with a function ofwavelength division multiplexing or bidirectional transmission has beenpermeating in order to realize high speed and advanced functions inpublic communication and computer networks.

[0003] In the field of optical communication, an optical integratedcircuit having various functions is actively studied in order to performan advanced optical signal processing. An optical integrated circuit hasan optical waveguide as a basic factor, which propagates light bycovering a core region having a high refractive index with a clad layerhaving a relatively low refractive index and thereby confining the lightin the core region and realizes various functions by pattering andarranging cores. Particularly, a quartz-based optical waveguide has manyadvantages such as small loss, physical chemical stability, andadjustability with an optical fiber and serves as a typical passiveoptical waveguide.

[0004] A typical optical-waveguide forming method uses the flamedeposition method as a core-clad-film forming method and thereactive-ion etching method as a core-pattern forming method. The CVDmethod, vacuum deposition method, and sputtering method are proposed ascore-clad forming methods in addition to the flame deposition method.

[0005] Though many methods are proposed as described above, there is notyet an optical-waveguide forming method having high performance, massproductivity, and low cost. This is because each of the film formingmethods has both advantage and disadvantage. For example, a high-qualitycore can be formed by the flame deposition method or CVD method.However, the flame deposition method requires high-temperature annealingat 1,000° C. or higher for more than ten hours a plurality of times andthe CVD method has a difficult point in mass production that afilm-forming area is narrow. Moreover, though the electron-beamdeposition method or sputtering method can realize small-loss filmformation, there is a problem in cost as an optical-waveguide formingprocess normally requiring a film thickness often to tens of micronsbecause a film forming rate is low.

[0006] To solve the above problems on formation of an optical waveguide,it is a very prospective optical-waveguide process to form a groove on asubstrate serving as a lower clad, to fill the groove with a materialhaving a higher refractive index than the substrate, and to use thegroove as a core because the core can be realized in a short time.

[0007] FIGS. 5(a) to 5(c) show the above type of optical-waveguideforming method. First, as shown in FIG. 5(a), a substrate 51 on which anoptical-waveguide groove is formed is filled with ahigh-refractive-index material 52 used as a core.

[0008] Then, as shown in FIG. 5(b), extra high-refractive-index materialis removed from the high-refractive-index material having been filled inthe step shown in FIG. 5(a).

[0009] Then, as shown in FIG. 5(c), a clad substrate 53 and theoptical-waveguide-groove substrate 51 are finally bonded together.

[0010] However, the above optical waveguide has the following problemson cost and performance.

[0011] In the case of the forming method of filling an optical-waveguidegroove shown in FIG. 5, if a high-refractive-index material is presentas an adhesive layer, the light confined in a core leaks to theadhesive-layer portion. Therefore, the step of removing extra corematerial when a core material is packed (FIG. 5(b)) is necessary and theformation cost is increased.

[0012] Moreover, to constitute a three-dimensional circuit, it isnecessary to remove extra material for each layer.

[0013] Furthermore, even if a step of removing extra material added, acore having a predetermined dimension cannot be obtained when theaccuracy of the step is not sufficient. However, it is difficult tocontrol the accuracy.

[0014] Moreover, when the extra material is not completely removed, itremains as a waveguide layer. Particularly, when polishing, finescratches are produced at a waveguide portion, and the scratches causewave-guiding light to scatter. Therefore, the present situation has aproblem in that an optical waveguide is not suitable for mass productionand it is difficult to reduce the cost of the waveguide.

[0015] That is, a conventional optical waveguide has a problem that thewaveguide is not suitable for mass production and it is difficult toreduce the cost of the waveguide.

DISCLOSURE OF THE INVENTION

[0016] The present invention is made to solve the above problems of aconventional optical waveguide and its object is to provide an opticalwaveguide having high performance, mass productivity, and low cost andan optical-waveguide forming method.

[0017] To solve the above problem, a first invention (corresponding toclaim 1) is an optical waveguide comprising:

[0018] a first substrate on which an optical-waveguide groove is formedand a second substrate, characterized in that

[0019] said second substrate is bonded to a plane of said firstsubstrate on which said optical-waveguide groove is formed by a materialhaving a refractive index higher than those of said first and secondsubstrates,

[0020] said optical-waveguide groove is filled with said material, and

[0021] the refractive index of said first substrate is different fromthat of said second substrate.

[0022] A second invention of the present invention (corresponding toclaim 2) is the optical waveguide according to the first invention ofthe present invention, characterized in that

[0023] by assuming the refractive index of said material as Na, that ofsaid first substrate as Nb, that of said second substrate as Nc, thethickness of said material present between said second substrate and aportion other than said optical-waveguide groove on the plane on whichthe optical-waveguide groove of said first substrate is formed as d, thedepth of said optical-waveguide groove as h, the wavelength ofwave-guiding light as λ,

[0024] (1) when said Nb is not smaller than said Nc, said Na, Nb, Nc, d,h, and λ simultaneously satisfy the following numerical formulas,$\begin{matrix}{d \leqq {\frac{\lambda}{2\quad \pi \sqrt{{N\quad a^{2}} - {N\quad b^{2}}}}{\tan^{- 1}\left( \frac{\sqrt{{N\quad b^{2}} - {N\quad c^{2}}}}{\sqrt{{N\quad a^{2}} - {N\quad b^{2}}}} \right)}}} & \left\lbrack {{Numerical}\quad {Formula}\quad 1} \right\rbrack\end{matrix}$

$\begin{matrix}{{{\frac{\lambda}{2\quad \pi \sqrt{{N\quad a^{2}} - {N\quad b^{2}}}}{\tan^{- 1}\left( \frac{\sqrt{{N\quad b^{2}} - {N\quad c^{2}}}}{\sqrt{{N\quad a^{2}} - {N\quad b^{2}}}} \right)}} < h}{and}} & \left\lbrack {{Numerical}\quad {Formula}\quad 2} \right\rbrack\end{matrix}$

[0025] (2) when said Nb is smaller than said Nc, said Na, Nb, Nc, d, h,and λ simultaneously satisfy the following numerical formulas.$\begin{matrix}{d \leqq {\frac{\lambda}{2\quad \pi \sqrt{{N\quad a^{2}} - {N\quad c^{2}}}}{\tan^{- 1}\left( \frac{\sqrt{{N\quad c^{2}} - {N\quad b^{2}}}}{\sqrt{{N\quad a^{2}} - {N\quad c^{2}}}} \right)}}} & \left\lbrack {{Numerical}\quad {Formula}\quad 3} \right\rbrack\end{matrix}$

$\begin{matrix}{{\frac{\lambda}{2\quad \pi \sqrt{{N\quad a^{2}} - {N\quad c^{2}}}}{\tan^{- 1}\left( \frac{\sqrt{{N\quad c^{2}} - {N\quad b^{2}}}}{\sqrt{{N\quad a^{2}} - {N\quad c^{2}}}} \right)}} < h} & \left\lbrack {{Numerical}\quad {Formula}\quad 4} \right\rbrack\end{matrix}$

[0026] A third invention of the present invention (corresponding toclaim 3) is the optical waveguide according to the first invention ofthe present invention, characterized in that

[0027] an optical-waveguide groove is also formed on the plane of saidsecond substrate bonded to said first substrate, and

[0028] the optical waveguide of said second substrate is filled withsaid material.

[0029] A fourth invention of the present invention (corresponding toclaim 4) is the optical waveguide according to the first invention ofthe present invention, characterized in that

[0030] a third substrate bonded by said material is provided for theplane opposite to the plane of said second substrate bonded to saidfirst substrate,

[0031] an optical-waveguide groove is also formed on the plane of saidthird substrate bonded to said second substrate,

[0032] the optical-waveguide groove of said third substrate is filledwith said material,

[0033] said material has a refractive index higher than that of saidthird substrate, and

[0034] the refractive index of said second substrate is different fromthe refractive index of said third substrate.

[0035] A fifth invention of the present invention (corresponding toclaim 5) is the optical waveguide according to any one of the first tothe fourth inventions of the present invention, characterized in that

[0036] said material is a glass-based material or resin.

[0037] A sixth invention of the present invention (corresponding toclaim 6) is the optical waveguide according to the fifth invention ofthe present invention, characterized in that

[0038] said material is a photo-curing resin, and

[0039] the expression of bonding together by said material denotesbonding together by applying light to said photo-curing resin and curingthe resin.

[0040] A seventh invention of the present invention (corresponding toclaim 7) is the optical waveguide according to any one of the first tothe fourth inventions of the present invention, characterized in that

[0041] said substrate is formed by a glass-based material or a resin.

[0042] An eighth invention of the present invention (corresponding toclaim 8) is the optical waveguide according to any one of the first tothe fourth inventions of the present invention, characterized in that

[0043] concave and convex portions of said optical-waveguide groove arecollectively formed through molding by a mold material having concaveand convex portions on its surface.

[0044] A ninth invention of the present invention (corresponding toclaim 9) is an optical-waveguide forming method of forming an opticalwaveguide having at least a first substrate and a second substratebonded to said first substrate, in which an optical-waveguide groove isformed on the bonded plane of at least either of said first and secondsubstrates, comprising:

[0045] a step of making the refractive index of said first substratedifferent from the refractive index of said second substrate by applyinglight to at least either of said first and second substrates and therebychanging refractive indexes; and

[0046] a step of bonding said first and second substrates different fromeach other in refractive index together by a material having arefractive index higher than refractive indexes of said first and secondsubstrates.

[0047] A tenth invention of the present invention (corresponding toclaim 10) is an optical-waveguide forming method of forming an opticalwaveguide having at least a first substrate and a second substratebonded to said first substrate, in which an optical-waveguide groove isformed on the bonded plane of at least either of said first and secondsubstrates, comprising:

[0048] a step of making the refractive index of said first substratedifferent from the refractive index of said second substrate by heatingand cooling at least either of said first or second substrates andthereby changing refractive indexes, and

[0049] a step of bonding said first and second substrates whoserefractive indexes are made different from each other by a materialhaving a refractive index higher than those of said first and secondsubstrates.

[0050] An eleventh invention of the present invention (corresponding toclaim 11) is the optical-waveguide forming method according to the ninthor the tenth invention of the present invention, characterized in that

[0051] by assuming the refractive index of said material as Na, that ofsaid first substrate as Nb, that of said second substrate as Nc, thethickness of said material present between said second substrate and aportion other than said optical-waveguide groove on the plane on whichthe optical-waveguide groove of said first substrate is formed as d, thedepth of said optical-waveguide groove as h, the wavelength ofwave-guiding light as λ,

[0052] (1) when said Nb is not smaller than said Nc, said Na, Nb, Nc, d,h, and λ simultaneously satisfy the following numerical formulas,$\begin{matrix}{d \leqq {\frac{\lambda}{2\quad \pi \sqrt{{N\quad a^{2}} - {N\quad b^{2}}}}{\tan^{- 1}\left( \frac{\sqrt{{N\quad b^{2}} - {N\quad c^{2}}}}{\sqrt{{N\quad a^{2}} - {N\quad b^{2}}}} \right)}}} & \left\lbrack {{Numerical}\quad {Formula}\quad 1} \right\rbrack\end{matrix}$

$\begin{matrix}{{{\frac{\lambda}{2\quad \pi \sqrt{{N\quad a^{2}} - {N\quad b^{2}}}}{\tan^{- 1}\left( \frac{\sqrt{{N\quad b^{2}} - {N\quad c^{2}}}}{\sqrt{{N\quad a^{2}} - {N\quad b^{2}}}} \right)}} < h}{and}} & \left\lbrack {{Numerical}\quad {Formula}\quad 2} \right\rbrack\end{matrix}$

[0053] (2) when said Nb is smaller than said Nc, said Na, Nb, Nc, d, h,and λ simultaneously satisfy the following numerical formulas.$\begin{matrix}{d \leqq {\frac{\lambda}{2\quad \pi \sqrt{{N\quad a^{2}} - {N\quad c^{2}}}}{\tan^{- 1}\left( \frac{\sqrt{{N\quad c^{2}} - {N\quad b^{2}}}}{\sqrt{{N\quad a^{2}} - {N\quad c^{2}}}} \right)}}} & \left\lbrack {{Numerical}\quad {Formula}\quad 3} \right\rbrack\end{matrix}$

$\begin{matrix}{{\frac{\lambda}{2\quad \pi \sqrt{{N\quad a^{2}} - {N\quad c^{2}}}}{\tan^{- 1}\left( \frac{\sqrt{{N\quad c^{2}} - {N\quad b^{2}}}}{\sqrt{{N\quad a^{2}} - {N\quad c^{2}}}} \right)}} < h} & \left\lbrack {{Numerical}\quad {Formula}\quad 4} \right\rbrack\end{matrix}$

[0054] A twelfth invention of the present invention (corresponding toclaim 12) is the optical-waveguide forming method according to the tenthinvention of the present invention, characterized in that

[0055] when making the refractive index of said first substratedifferent from the refractive index of said second substrate,

[0056] even the other-hand substrate is heated at a temperaturedifferent from the case of one-hand substrate to change refractiveindexes.

[0057] A thirteenth invention of the present invention (corresponding toclaim 13) is the optical-waveguide forming method according to the tenthinvention of the present invention), characterized in that

[0058] when making the refractive index of said first substratedifferent from the refractive index of said second substrate,

[0059] not only one-hand substrate but also the other-hand substrate areheated and cooled for periods different from each other to changerefractive indexes.

[0060] A fourteenth invention of the present invention (corresponding toclaim 14) is the optical-waveguide forming method according to the ninthor the tenth invention of the present invention, characterized in that

[0061] at least either of said first and second substrates is heated andsoftened to form said optical-waveguide groove by pressing a moldmaterial having concave and convex portions on the surface against thesubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0062]FIG. 1 is an illustration showing an optical waveguide of a firstembodiment of the present invention;

[0063]FIG. 2(a) is an illustration showing a relation betweenadhesive-layer thickness and effective refractive index calculated byassuming the refractive index of a waveguide core as 1.453, that of aclad substrate 13 as 1.45, and that of an optical waveguide substrate 11as 1.45 equal to the refractive index of the clad substrate 13 of thefirst embodiment of the present invention ;

[0064]FIG. 2(b) is an illustration showing a relation betweenadhesive-layer thickness and effective refractive index when increasingthe refractive index of the clad substrate 13 of the first embodiment ofthe present invention by 0.001 from the example shown in FIG. 2(a);

[0065]FIG. 2(c) is an illustration showing a relation betweenadhesive-layer thickness and effective refractive index when decreasingthe refractive index of the clad substrate 13 of the first embodiment ofthe present invention by 0.001 from the example shown in FIG. 2(a);

[0066]FIG. 3 is an illustration showing an optical waveguide of a fourthembodiment of the present invention;

[0067]FIG. 4 is an illustration showing an optical waveguide of a fifthembodiment of the present invention;

[0068]FIG. 5(a) is an illustration showing a step of filling the opticalwaveguide-groom substrate 51 on which an optical-waveguide groove isformed with the high-refractive-index material 52 used as a core in aconventional optical-waveguide forming method;

[0069]FIG. 5(b) is an illustration showing a step of removing extrahigh-refractive-index material from the high-refractive-index materialhaving been filled in the step shown in FIG. 5(a) in a conventionaloptical-waveguide forming method;

[0070]FIG. 5(c) is an illustration showing a step of bonding the cladsubstrate 53 and the optical-waveguide-groove substrate 51 together in aconventional optical-waveguide forming method;

[0071]FIG. 6(a) is an illustration showing a step of forming anoptical-waveguide groove through molding using a mold on the surface ofthe substrate 11 in an optical-waveguide forming method of a secondembodiment of the present invention;

[0072]FIG. 6(b) is an illustration showing a step of irradiating thesubstrate 11 on which an optical-waveguide groove is formed withultraviolet radiation in the optical-waveguide forming method of thesecond embodiment;

[0073]FIG. 6(c) is an illustration showing a step of applying ahigh-refractive-index material 12 to an optical-waveguide forming planeand filling a groove with the material 12 in the optical-waveguideforming method of the second embodiment;

[0074]FIG. 6(d) is an illustration showing a step of bonding theoptical-waveguide-groove substrate 11 irradiated with ultravioletradiation and the clad substrate 13 not irradiated with ultravioletradiation together in the optical-waveguide forming method of the secondembodiment;

[0075]FIG. 7 is an illustration sowing a parameter of the opticalwaveguide of the first embodiment of the present invention;

[0076]FIG. 8 is an illustration showing a relation betweenadhesive-layer thickness and effective refractive index calculated byassuming the refractive index of a waveguide core as 1.507, that of aclad substrate 13 as 1.504, and that of an optical waveguide groovesubstrate 11 as 1.504 equal to the refractive index of the cladsubstrate 13 of the first embodiment of the present invention;

[0077]FIG. 9(a) is an illustration showing a relation between adhesivelayer thickness and effective refractive index when increasing therefractive index of the clad substrate 13 of the first embodiment of thepresent invention by 0.001 from the example shown in FIG. 8;

[0078]FIG. 9(b) is an illustration showing a relation between adhesivelayer thickness and effective refractive index when further increasingthe refractive index of the clad substrate 13 of the first embodiment ofthe present invention by 0.001 from the example shown in FIG. 9(a);

[0079]FIG. 10(a) is an illustration showing a relation between adhesivelayer thickness and effective refractive index when decreasing therefractive index of the clad substrate 13 of the first embodiment of thepresent invention by 0.001 from the example shown in FIG. 8; and

[0080]FIG. 10(b) is an illustration showing a relation between adhesivelayer thickness and effective refractive index when further decreasingthe refractive index of the clad substrate 13 of the first embodiment ofthe present invention by 0.001 from the example shown in FIG. 9(a).

DESCRIPTION OF SYMBOLS

[0081]11, 51 . . . Optical-waveguide-groove substrate

[0082]31, 41 . . . First optical-waveguide-groove substrate

[0083]12, 32, 42, 52 . . . High-refractive-index material

[0084]13, 33, 44, 53 . . . Clad substrate

[0085]33, 43 . . . Second optical waveguide substrate

BEST MODE FOR CARRYING OUT THE INVENTION

[0086] Embodiments of the present invention are described below byreferring to accompanying drawings. Components provided with the samereference numeral in the drawings denote the same components.

[0087] First Embodiment

[0088]FIG. 1 shows the optical waveguide of the first embodiment of thepresent invention.

[0089] First, an optical-waveguide groove is formed on the surface ofthe substrate 11 made of glass or transparent resin through moldingusing a mold (not illustrated) as shown in FIG. 1.

[0090] Then, an ultraviolet-curing resin is applied to anoptical-waveguide-groove forming plane as the high-refractive-indexmaterial 12 and filled into a groove. Then, the optical-waveguide-groovesubstrate 11 and the clad substrate 13 having a refractive indexdifferent from that of the substrate 11 are bonded together. Theultraviolet-curing resin is applied by means of spin coating at arevolving speed of 500 to 7,000 rpm. The ultraviolet-curing resin in thegroove is cured through irradiation with ultraviolet radiation. By usinga resin having a refractive index higher than those of theoptical-waveguide-groove substrate 11 and clad substrate 13 as anultraviolet-curing resin, the ultraviolet-curing resin in the groovefunctions as an optical-waveguide core. As a result of observing theadhesive layer of the optical waveguide thus formed with an opticalmicroscope, it is found that the thickness of the adhesive layer isapprox. 1 μm.

[0091] FIGS. 2(a) to 2(c) show effects obtained by changing refractiveindexes of the clad substrate 13 and optical-waveguide-groove substrate11 by using a result of simulation. In FIGS. 2 (a) to 2(c), the axis ofabscissa shows a thickness of an adhesive layer and the axis of ordinateshows an effective refractive index, and a point at which the continuousline of a graph intersects with the axis of abscissa shows a cutoff filmthickness. When the thickness of the adhesive layer is smaller than thecutoff film thickness, a wave-guiding mode is not present. However, awavelength is set to 1.3 μm in every calculation.

[0092]FIG. 2(a) shows a result of performing calculation by assuming therefractive index of a waveguide core as 1.453, that of the cladsubstrate 13 as 1.45, and that of the optical-waveguide-groove 11 as1.45 equal to the refractive index of the clad substrate 13. In thiscase, because the continuous line intersects at the origin, awave-guiding mode is present even if an adhesive layer is made thin andlight propagates through the adhesive layer.

[0093] However, when increasing the refractive index of the cladsubstrate 13 to 1.451 by 0.001, a cutoff film thickness becomes 1.8 μmas shown in FIG. 2(b) and thus, no wave is guided as long as thethickness is 1.8 μm or less even if an adhesive layer is present.

[0094] Moreover, when decreasing the refractive index of the clad layer13 to 1.449 by 0.001, a cutoff film thickness becomes 1.2 μm as shown inFIG. 2(c) and no wave is guided as long as the thickness is 1.2 μm orless even if an adhesive layer is present similarly to the case ofincreasing the refractive index of the clad substrate 13.

[0095] Therefore, by changing refractive indexes of theoptical-waveguide-groove substrate 11 and clad substrate 13, it ispossible to prevent light from leaking even if an adhesive-layerremoving step is not executed.

[0096] Thus, a condition necessary to prevent light from leaking even ifthe adhesive-layer removing step is not executed is more minutelystudied through simulation. As a result, it is found that the followingis necessary.

[0097] That is, as shown in FIG. 7, the refractive index of theoptical-waveguide-groove substrate 11 is assumed as Nb, the depth of theoptical-waveguide groove formed on the optical-waveguide-groovesubstrate 11 is assumed as h, the refractive index of thehigh-refractive-index material 12 is assumed as Na, and the refractiveindex of the clad substrate 13 is assumed as Nc. Moreover, the thicknessof an adhesive layer is assumed as d and the wavelength of wave-guidinglight is assumed as λ.

[0098] Thus, when Na is larger than Nb and Nb is larger than Nc, it isfound that these parameters should simultaneously satisfy the followingnumerical formulas 1 and 2. $\begin{matrix}{d \leqq {\frac{\lambda}{2\quad \pi \sqrt{{N\quad a^{2}} - {N\quad b^{2}}}}{\tan^{- 1}\left( \frac{\sqrt{{N\quad b^{2}} - {N\quad c^{2}}}}{\sqrt{{N\quad a^{2}} - {N\quad b^{2}}}} \right)}}} & \left\lbrack {{Numerical}\quad {Formula}\quad 1} \right\rbrack\end{matrix}$

$\begin{matrix}{{\frac{\lambda}{2\quad \pi \sqrt{{N\quad a^{2}} - {N\quad b^{2}}}}{\tan^{- 1}\left( \frac{\sqrt{{N\quad b^{2}} - {N\quad c^{2}}}}{\sqrt{{N\quad a^{2}} - {N\quad b^{2}}}} \right)}} < h} & \left\lbrack {{Numerical}\quad {Formula}\quad 2} \right\rbrack\end{matrix}$

[0099] That is, when these parameters satisfy the above numericalformulas 1 and 2, light does not leak to an adhesive layer even if theadhesive-layer removing step is not executed.

[0100] Moreover, when Na is larger than Nc and Nc is larger than Nb, itis found that these parameters should satisfy the following numericalformulas 3 and 4. $\begin{matrix}{d \leqq {\frac{\lambda}{2\quad \pi \sqrt{{N\quad a^{2}} - {N\quad c^{2}}}}{\tan^{- 1}\left( \frac{\sqrt{{N\quad c^{2}} - {N\quad b^{2}}}}{\sqrt{{N\quad a^{2}} - {N\quad c^{2}}}} \right)}}} & \left\lbrack {{Numerical}\quad {Formula}\quad 3} \right\rbrack\end{matrix}$

$\begin{matrix}{{\frac{\lambda}{2\quad \pi \sqrt{{N\quad a^{2}} - {N\quad c^{2}}}}{\tan^{- 1}\left( \frac{\sqrt{{N\quad c^{2}} - {N\quad b^{2}}}}{\sqrt{{N\quad a^{2}} - {N\quad c^{2}}}} \right)}} < h} & \left\lbrack {{Numerical}\quad {Formula}\quad 4} \right\rbrack\end{matrix}$

[0101] That is, when these parameters simultaneously satisfy thenumerical formulas 3 and 4, light does not leak to the adhesive layereven if the adhesive-layer removing step is not executed.

[0102] Specifically, assuming Na as 1.507, Nc as 1.504, d as 1 μm, h as8 μm, and λ as 1.3 μm, and when Nb smaller than Na and larger than Nc isnot smaller than 1.5045 and it is smaller than 1.50664, wave-guidinglight does not leak to the adhesive layer.

[0103] Furthermore, assuming Na as 1.507, Nc as 1.503, d as 1 μm, h as 8μm, and λ as 1.3 μm, and when Nb smaller than Na and larger than Nc isnot smaller than 1.50383 and it is smaller than 1.50664, wave-guidinglight does not leak to the adhesive layer.

[0104] Thus, when the above conditions are satisfied, wave-guiding lightdoes not leak to the adhesive layer even if the thickness d of theadhesive layer is larger than zero.

[0105]FIG. 8 shows a relation between adhesive-layer thickness andeffective refractive index when h is 8 μm, λ is 1.3 μm, Na is 1.507, Nbis 1.504, and Nc is 1.504. In this case, because a continuous lineintersects at the origin, a wave-guiding mode is present even if theadhesive layer is made thin and light propagates through the adhesivelayer.

[0106]FIG. 9(a) shows a relation between adhesive-layer thickness andeffective refractive index when increasing Nc by 0.001. That is, FIG.9(a) shows a relation between adhesive-layer thickness and effectiverefractive index when h is 8 μm, λ is 1.3 μm, Na is 1.507, Nb is 1.504,and Nc is 1.505.

[0107] In this case, the cutoff film thickness becomes 1.8 μm , and nowave is guided when a cutoff film thickness is 1.8 μm or less even if anadhesive layer is present.

[0108] Moreover, FIG. 9(b) shows a case in which Nc is further increasedby 0.001 from the case of FIG. 9(a). That is, FIG. 9(b) shows a relationbetween adhesive-layer thickness and effective refractive index when his 8 μm, λ is 1.3 μm, Na is 1.507, Nb is 1.504, and Nc is 1.506.

[0109] In this case, a cutoff film thickness becomes 3.8 μm but no waveis guided when the thickness is 3.8 μm or less even if an adhesive layeris present. That is, when Nb is larger than Nc, wave-guiding light doesnot propagate through the adhesive layer even if the adhesive layerbecomes thicker if the difference between refractive indexes of Nc andNb increases.

[0110]FIG. 10(a) shows a relation between adhesive-layer pressure andeffective refractive index when decreasing Nc by 0.001 compared to thecase of FIG. 8. That is, FIG. 10(a) shows a relation betweenadhesive-layer thickness and effective refractive index when h is 8 μm,λ is 1.3 μm, Na is 1.507, Nb is 1.504, and Nc is 1.503.

[0111] In this case, the cutoff film thickness becomes 1.3 μm and nowave is guided when the thickness is 1.3 μm or less even if the adhesivelayer is present.

[0112] Furthermore, FIG. 10(b) shows a case in which Nc is decreased by0.001 from the case of FIG. 10(a). That is, FIG. 10(b) shows a relationbetween adhesive-layer thickness and effective refractive index when his 8 μm, λ is 1.3 μm Na is 1.507, Nb is 1.504, and Nc is 1.502.

[0113] In this case, the cutoff film thickness becomes 1.6 μm and nowave is guided when a cutoff film thickness is 1.6 μm or less even if anadhesive layer is present.

[0114] Thus, also when Nc is larger than Nb, it can be said thatwave-guiding light does not propagate through an adhesive layer even ifthe thickness of the adhesive layer further increases when thedifference in refractive index increases.

[0115] Thus, it is possible to obtain an optical waveguide in whichwave-guiding light does not leak to an adhesive layer even if theadhesive layer is present by selecting conditions satisfying thenumerical formulas 1 and 2 when Na is larger than Nb and Nb is largerthan Nc and those satisfying the numerical formulas 3 and 4 when Na islarger than Nc and Nc is larger than Nb as parameters shown in FIG. 7.

[0116] The optical-waveguide-groove substrate 11 of this embodimentserves as a first substrate of the present invention and the cladsubstrate 13 of this embodiment serves as a second substrate of thepresent invention.

[0117] Moreover, the second substrate of the present invention is notrestricted to a substrate of changing the refractive index of the cladsubstrate 13 as shown in this embodiment. In short, it is only necessarythat the second substrate of the present invention is different from thefirst substrate of the present invention in refractive index such as asubstrate changing the refractive index of the optical-waveguide-groovesubstrate 11.

[0118] Furthermore, though a refractive-index change is calculated as0.001 in the case of this embodiment, the change is not restricted to0.001. It is allowed to change a refractive index as long as refractiveindexes of the clad substrate 13 and optical-waveguide-groove substrate11 do not exceed the refractive index of a core.

[0119] Furthermore, though a wavelength is calculated as 1.3 μm in thecase of this embodiment, the wavelength is not restricted to 1.3 μm.Other wavelengths are also considered by generalizing them.

[0120] Furthermore, though an ultraviolet-curing resin is used as anoptical-waveguide core material in the case of this embodiment, the corematerial is not restricted to the resin. It is also allowed to use athermosetting resin or glass-based material.

[0121] Furthermore, though spin coating is used as a method of coating ahigh-refractive-index material in the case of this embodiment, thecoating method is not restricted to the spin coating. It is allowed touse dip coating or spray coating.

[0122] Furthermore, though it is preferable to form an optical waveguidegroove through molding as described for this embodiment from theviewpoint of productivity, formation of the groove is not restricted tomolding. It is allowed to form the groove through etching according tonecessity.

[0123] Furthermore, though one linear waveguide is described as anexample, this embodiment, is not restricted to the linear waveguide. Itis possible to apply this embodiment to every optical waveguide patterngenerally used and moreover control bend, branch, and combination ofoptical waves.

[0124] Second Embodiment

[0125] Then, the optical-waveguide forming method of the secondembodiment of the present invention is described below.

[0126] FIGS. 6(a) to 6(d) show the optical-waveguide forming method ofthis embodiment.

[0127] As shown in FIG. 6(a), an optical-waveguide groove is formed onthe surface of a substrate 11 made of glass or transparent resin throughmolding using a mold (not illustrated). That is, the optical-waveguidegroove is formed by heating and thereby softening the substrate 11 andpressing a mold material having concave and convex portions on itssurface against the substrate 11.

[0128] Then, as shown in FIG. 6(b), ultraviolet radiation is applied tothe substrate 11 on whitch an optical-waveguide groove is formed. Byapplying ultraviolet radiation to the substrate, a refractive-indexchange is induced due to a photochemical reaction. Therefore, it ispossible to make the refractive index of the optical-waveguide-groovesubstrate 11 different from that of a clad substrate 13 even if usingthe same material for the substrates 11 and 13.

[0129] Then, as shown in FIG. 6(c), a high-refractive-index material 12is applied to an optical-waveguide-groove forming plane to fill a groovewith the material 12 as shown in FIG. 6(c).

[0130] Finally, as shown in FIG. 6(d), the optical-waveguide-groovesubstrate 11 irradiated with ultraviolet radiation and the cladsubstrate 13 not irradiated with ultraviolet radiation are bondedtogether.

[0131] Thus, it is possible to form the optical waveguide shown in FIG.1.

[0132] A first substrate of the present invention is not restricted tothe substrate of this embodiment changing the refractive index of thesubstrate 11 on which an optical-waveguide groove is formed like thecase of this embodiment. In short, such as changing the refractive indexof the clad substrate 13, it is only necessary that the first substrateof the present invention is different from a second substrate of thepresent invention in refractive index.

[0133] Moreover, the first substrate of the present invention is notrestricted to a substrate changing the refractive index of only thesubstrate 11 on which an optical waveguide groove is formed like thecase of this embodiment. In short, such as irradiating the cladsubstrate 13 and optical-waveguide-groove substrate 11 with ultravioletradiation by different amount of irradiation, it is only necessary thatthe first substrate of the present invention is different from thesecond substrate of the present invention in refractive index.

[0134] Furthermore, an optical-waveguide forming method of forming theoptical waveguide shown in FIG. 1 is described for this embodiment.However, this embodiment is not restricted to the above method. Theoptical-waveguide forming method of this embodiment makes it possible toform the optical waveguide shown in FIG. 3 or 4. The optical waveguidesshown in FIGS. 3 and 4 will be described later.

[0135] Furthermore, it is preferable to form an optical-waveguide groovethrough molding as described for this embodiment from the viewpoint ofproductivity. However, an optical-waveguide groove forming method is notrestricted to molding. However, it is also allowed to form anoptical-waveguide groove through etching according to necessity.

[0136] Third Embodiment

[0137] An optical-waveguide forming method of a third embodiment of thepresent invention is described below.

[0138] In the case of the optical-waveguide forming method of the secondembodiment, the substrate 11 is irradiated with ultraviolet radiation inthe step shown in FIG. 6(b). In the case of the third embodiment,however, a substrate 11 is heated and cooled instead.

[0139] That is, an optical waveguide groove is formed on the surface ofthe substrate 11 made of glass or transparent resin through moldingusing a mold (not illustrated) as shown in FIG. 1 similarly to the caseof FIG. 6(a) of the second embodiment.

[0140] Then, the substrate 11 with the optical waveguide groove formedis heated. Glass has a structure corresponding to its heat history andthe density of the glass at room temperature depends on a heatingtemperature or cooling rate. For example, rapidly-cooled glass has avolume larger than slowly-cooled glass. Therefore, the refractive indexof the former is different from that of the latter depending on aheating temperature or cooling rate. Therefore, it is possible to makethe refractive index of the optical-waveguide-groove substrate 11different from that of a clad substrate 13 even if using the samematerial for the both substrates 11 and 13.

[0141] Then, an ultraviolet-curing resin is applied to anoptical-waveguide-groove forming plane as a high-refractive-indexmaterial 12 to fill a groove with the resin.

[0142] Thereafter, the optical-waveguide-groove substrate 11 and theclad substrate 13 are bonded together. By applying ultravioletradiation, the ultraviolet-curing resin in the groove is cured. By usinga material having a refractive index higher than that of theoptical-waveguide-groove substrate 11 and upper clad substrate 13 as anultraviolet-curing resin, the ultraviolet-curing resin in the groovefunctions as an optical-waveguide core.

[0143] In the case of this embodiment, refractive indexes are changed byheating a substrate with an optical waveguide groove formed. It is onlynecessary that the clad substrate 13 is different from theoptical-waveguide-groove substrate 11 in refractive index, and it isallowed to change the refractive index of the clad substrate 13.

[0144] Moreover, though refractive indexes are changed by heating onlythe substrate 11 with the optical-waveguide groove formed in the case ofthis embodiment, it is also allowed to heat two substrates at differenttemperatures.

[0145] Furthermore, though refractive indexes are changed by heatingonly the substrate 11 with the optical-waveguide groove formed in thecase of this embodiment, it is also allowed to heat two substrates atthe same temperature and cool them by changing cooling rates.

[0146] Furthermore, though an ultraviolet-curing resin is used as anoptical-waveguide core material in the case of this embodiment, theoptical-waveguide core material is not restricted to the resin. It isallowed to use a glass-based material.

[0147] Furthermore, though it is preferable to form an optical waveguidegroove through molding as described for this embodiment from theviewpoint of productivity, an optical-waveguide-groove forming method isnot restricted to molding. It is also allowed to form theoptical-waveguide groove through etching according to necessity.

[0148] Fourth Embodiment

[0149] An optical waveguide of a fourth embodiment of the presentinvention is described below.

[0150]FIG. 3 is an illustration showing an optical waveguide of thefourth embodiment of the present invention.

[0151] As shown in FIG. 3, an optical-waveguide groove is formed on thesurface of a first substrate 31 made of glass or transparent resinthrough molding using a mold (not illustrated). Moreover, anoptical-waveguide groove is formed on the surface of a second substrate33 different from the first substrate 31 in refractive index throughmolding using a mold (not illustrated).

[0152] Then, an ultraviolet-curing resin is applied to anoptical-waveguide-groove forming plane as a high-refractive-indexmaterial 32 to fill a groove with the resin. Thereafter, the firstsubstrate 31 and the second substrate 33 are bonded together. Byapplying ultraviolet radiation, the ultraviolet-curing resin in thegroove is cured. By using a material having a refractive index higherthan those of the first optical-waveguide-groove substrate 31 and secondoptical-waveguide-groove substrate 33 as an ultraviolet-curing resin,the ultraviolet-curing resin in the groove functions as anoptical-waveguide core. By using the above configuration, it is possibleto fabricate an optical circuit having a higher integration degree.Moreover, by overlapping parts of a waveguide and bonding them together,it is possible to use the waveguide as a three-dimensional crossed orbranched waveguide.

[0153] The first optical-waveguide-groove substrate 31 of thisembodiment serves as a first substrate of the present invention and thesecond optical-waveguide-groove substrate 33 of this embodiment servesas a second substrate of the present invention.

[0154] Moreover, though this embodiment uses a substrate having arefractive index different from that of the first substrate as thesecond substrate, it is also allowed to form a groove by using the samesubstrate and then change refractive indexes through heat treatment orirradiation with light.

[0155] Furthermore, though this embodiment uses an ultraviolet-curingresin as an optical-waveguide core material, the core material is notrestricted to the resin. It is also allowed to use a thermosetting resinor glass-based material.

[0156] Furthermore, it is preferable to form an optical-waveguide groovethrough molding as described for this embodiment. However, a method offorming the optical-waveguide groove is not restricted to molding. It isallowed to form the groove through etching according to necessity.

[0157] Furthermore, though a pair of waveguides is described for thisembodiment as an example, the number of waveguides is not restricted toa pair of waveguides. It is also allowed to form a plurality ofwaveguides on one substrate.

[0158] Furthermore, though a linear waveguide is described for thisembodiment as an example, this embodiment is not restricted to thelinear waveguide. It is possible to apply this embodiment to everyoptical-waveguide pattern generally used and moreover control bend,branch, and combination of optical waves.

[0159] Fifth Embodiment

[0160] Then, the fifth embodiment is described below.

[0161]FIG. 4 is an optical waveguide of the fourth embodiment of thepresent invention.

[0162] As shown in FIG. 4, an optical-waveguide groove is formed on thesurface of a first substrate 41 made of glass or transparent resinthrough molding using a mold (not illustrated). Moreover, anoptical-waveguide groove is formed on surfaces of a first substrate 41and a second substrate 43 through molding using a mold (notillustrated).

[0163] Then, an ultraviolet-curing resin is applied to anoptical-waveguide-groove forming plane as a high-refractive-indexmaterial 42 to fill a groove with the resin. Thereafter, the firstoptical-waveguide-groove substrate 41 and secondoptical-waveguide-groove substrate 43 are bonded together through a cladsubstrate 44 different from the first optical-waveguide-groove substrate41 and second optical-waveguide-groove substrate 43 in refractive indexso that the optical-waveguide grooves are faced each other. By applyingultraviolet radiation, the ultraviolet-curing resin in the groove iscured. By using a material having a refractive index higher than thoseof the first optical-waveguide-groove substrate 41, secondoptical-waveguide-groove substrate 43, and clad substrate 44 as anultraviolet-curing resin, the ultraviolet-curing resin in the groovefunctions as an optical-waveguide core. By setting the thickness of theclad substrate to several microns, it is possible to easily form athree-dimensional directional coupler.

[0164] The first optical-waveguide-groove substrate 41 of thisembodiment serves as a first substrate of the present invention, thesecond optical-waveguide-groove substrate 43 of this embodiment servesas a third substrate of the present invention, and the clad substrate 44of this embodiment serves as a second substrate of the presentinvention.

[0165] Moreover, though this embodiment uses an ultraviolet-curing resinas an optical-waveguide core material, the core material is notrestricted to the ultraviolet-curing resin. It is allowed to use athermosetting resin or glass-based material.

[0166] Furthermore, it is preferable to form an optical waveguide groovethrough molding as described for this embodiment from the viewpoint ofproductivity. However, a method of forming the groove is not restrictedto molding. It is allowed to form the groove through etching accordingto necessity.

[0167] Furthermore, though this embodiment uses a substrate having arefractive index different from those of the first and second substratesas a clad substrate. However, it is also allowed to use the samesubstrate by changing refractive indexes through a heat treatment orirradiation with light.

[0168] Furthermore, though separate clad substrates are bonded togetherin the case of this embodiment, it is also allowed to deposit a thinfilm instead of the above case.

[0169] Furthermore, though this embodiment is described by using a pairof waveguides as an example, it is allowed to form a plurality ofwaveguides on one substrate instead of the above mentioned.

[0170] Furthermore, though this embodiment is described by using alinear waveguide as an example, it is not restricted to the linearwaveguide. It is possible to apply this embodiment to every opticalwaveguide pattern and moreover control bend, branch, and combination ofoptical waves.

Industrial Applicability

[0171] As clearly shown from the described above, the present inventionprovides an optical waveguide which can be mass-produced at a low costand an optical-waveguide forming method.

1. An optical waveguide comprising: a first substrate on which anoptical-waveguide groove is formed and a second substrate, characterizedin that said second substrate is bonded to a plane of said firstsubstrate on which said optical-waveguide groove is formed by a materialhaving a refractive index higher than those of said first and secondsubstrates, said optical-waveguide groove is filled with said material,and the refractive index of said first substrate is different from thatof said second substrate.
 2. The optical waveguide according to claim 1,characterized in that by assuming the refractive index of said materialas Na, that of said first substrate as Nb, that of said second substrateas Nc, the thickness of said material present between said secondsubstrate and a portion other than said optical-waveguide groove on theplane on which the optical-waveguide groove of said first substrate isformed as d, the depth of said optical-waveguide groove as h, thewavelength of wave-guiding light as λ, (1) when said Nb is not smallerthan said Nc, said Na, Nb, Nc, d, h, and λ simultaneously satisfy thefollowing numerical formulas, $\begin{matrix}{d \leqq {\frac{\lambda}{2\quad \pi \sqrt{{N\quad a^{2}} - {N\quad b^{2}}}}{\tan^{- 1}\left( \frac{\sqrt{{N\quad b^{2}} - {N\quad c^{2}}}}{\sqrt{{N\quad a^{2}} - {N\quad b^{2}}}} \right)}}} & \left\lbrack {{Numerical}\quad {Formula}\quad 1} \right\rbrack\end{matrix}$

$\begin{matrix}{{{\frac{\lambda}{2\quad \pi \sqrt{{N\quad a^{2}} - {N\quad b^{2}}}}{\tan^{- 1}\left( \frac{\sqrt{{N\quad b^{2}} - {N\quad c^{2}}}}{\sqrt{{N\quad a^{2}} - {N\quad b^{2}}}} \right)}} < h}\quad {and}} & \left\lbrack {{Numerical}\quad {Formula}\quad 2} \right\rbrack\end{matrix}$

(2) when said Nb is smaller than said Nc, said Na, Nb, Nc, d, h, and λsimultaneously satisfy the following numerical formulas. $\begin{matrix}{d \leqq {\frac{\lambda}{2\quad \pi \sqrt{{N\quad a^{2}} - {N\quad c^{2}}}}{\tan^{- 1}\left( \frac{\sqrt{{N\quad c^{2}} - {N\quad b^{2}}}}{\sqrt{{N\quad a^{2}} - {N\quad c^{2}}}} \right)}}} & \left\lbrack {{Numerical}\quad {Formula}\quad 3} \right\rbrack\end{matrix}$

$\begin{matrix}{{\frac{\lambda}{2\quad \pi \sqrt{{N\quad a^{2}} - {N\quad c^{2}}}}{\tan^{- 1}\left( \frac{\sqrt{{N\quad c^{2}} - {N\quad b^{2}}}}{\sqrt{{N\quad a^{2}} - {N\quad c^{2}}}} \right)}} < h} & \left\lbrack {{Numerical}\quad {Formula}\quad 4} \right\rbrack\end{matrix}$


3. The optical waveguide according to claim 1, characterized in that anoptical-waveguide groove is also formed on the plane of said secondsubstrate bonded to said first substrate, and the optical waveguide ofsaid second substrate is filled with said material.
 4. The opticalwaveguide according to claim 1, characterized in that a third substratebonded by said material is provided for the plane opposite to the planeof said second substrate bonded to said first substrate, anoptical-waveguide groove is also formed on the plane of said thirdsubstrate bonded to said second substrate, the optical-waveguide grooveof said third substrate is filled with said material, said material hasa refractive index higher than that of said third substrate, and therefractive index of said second substrate is different from therefractive index of said third substrate.
 5. The optical waveguideaccording to any one of claims 1 to 4, characterized in that saidmaterial is a glass-based material or resin.
 6. The optical waveguideaccording to claim 5, characterized in that said material is aphoto-curing resin, and the expression of bonding together by saidmaterial denotes bonding together by applying light to said photo-curingresin and curing the resin.
 7. The optical waveguide according to anyone of claims 1 to 4, characterized in that said substrate is formed bya glass-based material or a resin.
 8. The optical waveguide according toany one of claims 1 to 4, characterized in that concave and convexportions of said optical-waveguide groove are collectively formedthrough molding by a mold material having concave and convex portions onits surface.
 9. An optical-waveguide forming method of forming anoptical waveguide having at least a first substrate and a secondsubstrate bonded to said first substrate, in which an optical-waveguidegroove is formed on the bonded plane of at least either of said firstand second substrates, comprising: a step of making the refractive indexof said first substrate different from the refractive index of saidsecond substrate by applying light to at least either of said first andsecond substrates and thereby changing refractive indexes; and a step ofbonding said first and second substrates different from each other inrefractive index together by a material having a refractive index higherthan refractive indexes of said first and second substrates.
 10. Anoptical-waveguide forming method of forming an optical waveguide havingat least a first substrate and a second substrate bonded to said firstsubstrate, in which an optical-waveguide groove is formed on the bondedplane of at least either of said first and second substrates,comprising: a step of making the refractive index of said firstsubstrate different from the refractive index of said second substrateby heating and cooling at least either of said first or secondsubstrates and thereby changing refractive indexes, and a step ofbonding said first and second substrates whose refractive indexes aremade different from each other by a material having a refractive indexhigher than those of said first and second substrates.
 11. Theoptical-waveguide forming method according to claim 9 or 10,characterized in that by assuming the refractive index of said materialas Na, that of said first substrate as Nb, that of said second substrateas Nc, the thickness of said material present between said secondsubstrate and a portion other than said optical-waveguide groove on theplane on which the optical-waveguide groove of said first substrate isformed as d, the depth of said optical-waveguide groove as h, thewavelength of wave-guiding light as λ, (1) when said Nb is not smallerthan said Nc, said Na, Nb, Nc, d, h, and 80 simultaneously satisfy thefollowing numerical formulas, $\begin{matrix}{d \leqq {\frac{\lambda}{2\quad \pi \sqrt{{N\quad a^{2}} - {N\quad b^{2}}}}{\tan^{- 1}\left( \frac{\sqrt{{N\quad b^{2}} - {N\quad c^{2}}}}{\sqrt{{N\quad a^{2}} - {N\quad b^{2}}}} \right)}}} & \left\lbrack {{Numerical}\quad {Formula}\quad 1} \right\rbrack\end{matrix}$

$\begin{matrix}{{{\frac{\lambda}{2\quad \pi \sqrt{{N\quad a^{2}} - {N\quad b^{2}}}}{\tan^{- 1}\left( \frac{\sqrt{{N\quad b^{2}} - {N\quad c^{2}}}}{\sqrt{{N\quad a^{2}} - {N\quad b^{2}}}} \right)}} < h}\quad {and}} & \left\lbrack {{Numerical}\quad {Formula}\quad 2} \right\rbrack\end{matrix}$

(2) when said Nb is smaller than said Nc, said Na, Nb, Nc, d, h, and λsimultaneously satisfy the following numerical formulas. $\begin{matrix}{d \leqq {\frac{\lambda}{2\quad \pi \sqrt{{N\quad a^{2}} - {N\quad c^{2}}}}{\tan^{- 1}\left( \frac{\sqrt{{N\quad c^{2}} - {N\quad b^{2}}}}{\sqrt{{N\quad a^{2}} - {N\quad c^{2}}}} \right)}}} & \left\lbrack {{Numerical}\quad {Formula}\quad 3} \right\rbrack\end{matrix}$

$\begin{matrix}{{\frac{\lambda}{2\quad \pi \sqrt{{N\quad a^{2}} - {N\quad c^{2}}}}{\tan^{- 1}\left( \frac{\sqrt{{N\quad c^{2}} - {N\quad b^{2}}}}{\sqrt{{N\quad a^{2}} - {N\quad c^{2}}}} \right)}} < h} & \left\lbrack {{Numerical}\quad {Formula}\quad 4} \right\rbrack\end{matrix}$


12. The optical-waveguide forming method according to claim 10,characterized in that when making the refractive index of said firstsubstrate different from the refractive index of said second substrate,even the other-hand substrate is heated at a temperature different fromthe case of one-hand substrate to change refractive indexes.
 13. Theoptical-waveguide forming method according to claim 10, characterized inthat when making the refractive index of said first substrate differentfrom the refractive index of said second substrate,